Generic methods are well known in prior art. An error detected in the volume of the test object by means of an impulse echo method based on radiating pulsed ultrasound into the test object, for example, a cavity, an inclusion or also a crack, are characterized by stating a value for its equivalent reflector size ERG. The value of this equivalent reflector size is determined by a comparison of the amplitudes of the echo signals that are caused by the tested error in the volume of the test object, with the model of a comparative error of known size. In the so-called reference standard method, the test operator compares the echo signals of the tested test object with the echo signals which he obtains at a reference standard that is equivalent to the test object, which has one or more standard reflectors. For example, for this purpose, cylindrical bores with known dimensions can be inserted into the reference standard. The echo signals occurring at the bore during an ultrasound reflection that are obtained while testing the test object, are compared. In the reference standard method, the test operator therefore uses a suitable probe, that can be, for example, a suitable angle probe, measurements at the test object that is to be tested, as well as on the prepared reference standard.
In contrast, in the so-called AVG method, the amplitude of an echo signal resulting from an error in the volume of the test object is compared with a theoretically calculated and/or empirically determined echo signal of a model reference error, which is assumed to be a level circular disk, as a rule, and which is at the same depth in the test object as the error detected during the test of the test object. For this purpose, a so-called AVG diagram is prepared in advance for the probe used in the test, which contains the characteristics of the probe. The curves contained in the AVG diagram indicate the echo amplitude, which would be created by a reference error when measuring with the probe that is used. In a practical test problem, the test operator and then read the equivalent reflector size of the error detected in the volume of the test object by making a sound-attenuation correction (material-specific sound attenuation) and transfer correction (test object-specific injection losses) for the test object directly off the AVG diagram.
In a classic test method according to the AVG method, the test operator varies the probe position and orientation relative to the error found and tries to thereby maximize the resulting echo signal. This process is also described as “breeding” the ultrasound signal when testing materials by means of ultrasound. The actual determination of the equivalent reflector size of the detected error then takes place for the maximized ultrasound echo.
Additional details of the AVG method result, for example, from patent specification U.S. Pat. No. 5,511,425 A, which goes back to the legal predecessor of the applicant. Furthermore, the AVG method is described in detail in the book “Material Testing with Ultrasound”, J. Krautkrämer and H. Krautkrämer, 5th edition, Springer Verlag, ISBN 3-540-15754-9, chapter 19.1, pages 343-349. The technical details concerning the AVG method revealed here are being added in their entirety to the revelation content of this application by means of this reference.
In its currently prevalent form, it is a disadvantage of the AVG method that for a meaningful characterization of an error in the volume of a test object, a test must be performed with a number of probes. This has its reasons therein, that for a given error, a perpendicular radiation into the test object does not necessarily supply a maximum echo amplitude. Rather, it depends on the orientation of the error in the volume of the test object at which irradiation angle a maximum echo signal can be obtained. In order to actually obtain a value for the equivalent reflector size of a detected error, that is reasonably correlated with the actual size of the error, therefore, as a rule, within the scope of standardized test procedures based on the AVG method, different angle probes are used which realize different irradiation angles. In practice, this method signifies significant testing and documentation effort for the test operator, so that as a rule, testing is only performed at a few irradiation angles. Beyond that the variation of the irradiation angle requires a change of the probe, which causes additional problems because of the never one hundred percent unambiguous calibration in addition to the connection properties of the probes.